Towards the end of 2009 many grouted connection joints, between large diameter monopiles and connecting tubular steel transition pieces at the base of overlying support towers, were failing.
On closer inspection, this appeared to be due to the extremely brittle grout used cracking up, due to tension stresses being present as a result of repeated tower bending. More worrying still, this was not an isolated incident. Dr. Chris Golightly, an experienced, independent offshore oil and gas and renewables industry, geotechnical and foundations consultant explains.
In the early days of offshore wind, the grouting technique was adopted as a supposed quicker and cheaper method of allowing turbine towers to be placed close to vertical on non-vertically installed (up to 1 degree) driven monopiles. It was sold as a quick, cost effective and apparently problem-free solution. Prior to this, bolted flanged connections had been used at the initial UK offshore sites such as Scroby Sands, North Hoyle and Blyth.
However, in October 2009 Shell first discovered this systemic design error at their Egmond aan Zee wind farm. The problem then rapidly escalated throughout much of the industry , soon afterwards.
Subsequently, it was discovered that over 70% of monopiles in European waters have been affected. This has necessitated expensive repairs, ranging from insertion of elastomeric spring bearings at Sheringham Shoal, to simply filling piles with mass concrete at other locations. In all, at least 18 projects involving in excess of 650 turbines have been affected.
It was then determined that the failures were primarily due to the widespread non-use of shear keys – a series of horizontal rings of welded steel beads, spaced a few tens of centimetres apart placed on the inner surface of the tubular steel transition piece and the outer monopile surface.
Shear keys act as very effective reinforcement to the grout and their exclusion is now considered to be a mistake. Those connections that did not fail included shear keys (such as at Arklow Bank, Robin Rigg and Barrow), where the designers were experienced in oil and gas platform design – an industry in which the use of shear keys has been common practice for decades.
However the grout in offshore oil and gas pile to jacket connections is usually always in compression, primarily because of the heavy axial dead weight of platforms. In wind turbine monopile connections though, the situation is different. Since severe lateral cyclic bending occurs during extreme wind and wave loading conditions, the dead weight is proportionally much lower and bending predominates.
As a result, this leads to periodic tensile stressing of the compressively strong but extremely brittle grouts, which eventually crack and crumble. This can result in failure, settlement, tower tilting and the structure frequently ending up resting on internal support brackets which are not designed for that purpose.
More broadly, the grout failures may have been linked to minor deviations in manufacturing and installation tolerances, leading to small changes in individual turbine installation configurations. Higher plasticity, lower strength, less brittle and fatigue resistant grouts are now available, although proposals to add steel fibres to the grout are still not considered feasible based upon extensive work carried out at Leibniz University, Hannover some years ago.
In any case, as the market pushed on, far too much credence was given to inappropriate “black box” numerical modelling. Ultimately, there were sufficiently knowledgeable and experienced people who would have recognised that “gluing” large diameter twin steel tubes of varying inclination together with cement grout that sets to an extremely strong and brittle rock like material was eventually likely to lead to failure.
Given the industry’s relative infancy, developers, owners and lenders engineers and certifiers in offshore wind are still relatively inexperienced in many aspects of offshore design, construction and installation. Organisations with the appropriate levels of experience and technical know-how have not always been engaged and there has been a tendency for project owners and lenders’ advisors not to “rock boats” in identifying potential offshore construction problems. There is an alarming asymmetry between construction risks and the number of players who can manage these risks effectively.
This is still the case in most instances in this nascent sector and, with far too much design responsibility being passed across to contractors, there is a culture of “box ticking” and over adherence to codes and standards and not enough good engineering design and creative, imaginative thinking.
In short, this simply has to change. The DNV J101 2007 design code left it open for designers not to use shear keys and many took that option ostensibly to save costs and because the necessity to use shear keys was, in many people’s eyes, still open for debate.
In a more recent development, DNV recommendations based upon a Joint Industry Project suggest that placing a slight taper of 1 to 3 degrees on the tubular steel transition piece will cater for the downward slippage when failure occurs as a result of grout cracking, thereby allowing the joint to go back into compression. Precisely how and when this will occur seems unclear.
This recommendation has been adopted at London Array, Walney 2 and Anholt, and whether or not it is sufficiently robust for the design lifetime remains to be seen.
Whatever the case, grouted connections for monopiles are now finally regarded as acceptable if correctly designed, with shear keys. Carefully monitored and accurate pile installation is essential, with the use of grout seals specifically tailored to varying and eccentric annulus thicknesses an absolute must.
For tripod/jacket structures where the legs are stabbed inside accurately positioned pre-driven piles (as undertaken at the Alpha Ventus and Ormonde, Irish Sea projects), grouted joints are now also considered acceptable. Although reported problems at the German Borkum West 2 piled tripod project, where grouted seals have failed during installation, suggest that structure legs placed over the outside of pre-installed pile groups may not yet be regarded as an adequate solution.
So is grout necessary and do alternative solutions exist?
For many within the market, it’s the million-dollar question – and it’s certainly something that’s come increasingly to the fore, as design and innovation moves on.
One viable alternative is the bolted flange. Although it requires more stringent tower verticality, cannot be damaged during the pile driving and will need to overcome issues such as water-tightness and the location of boat landing attachments, the potential remains. Particularly as the cost differential between grout and bolted connections are relatively minor.
Other options include slip joints and hydra-lok mechanical connectors – all of which are currently being considered.
It is still likely that several monopile projects will no doubt continue to adopt grouting, but given the recent publicity and its implications for future project costs, many developers are already beginning to make the switch.
As such, the question investors, lenders and insurers are now asking is: why should we take the risk with grout, in view of what has happened, if there are better alternatives?
Dr. Chris Golightly is a foundation engineering expert and has provided advice on offshore grouted connection joints. Dr. Golightly, formerly of Fugro Engineers, BP and Shell, has been working as an independent consultant since October 2005.
Dr. Golightly has focused on offshore wind and tidal energy since 2010 and has undertaken work for several German offshore wind farms and review work for UK Round 3 sites, amongst other projects. Current clients include developers, consultants, certification agencies, insurance underwriters and government funded/private research organisations.
All views expressed in this article are those of the author, not those of A Word About Wind.